It depends on what you mean by sustainable, it can mean a lot of different things.

In terms of CO2 production, plastics really can't be beat at the moment. Their weight relative to other materials makes them cost significantly less CO2 to ship.

In terms of biodegradability, we have composable plastics like PLA but you're not going to get the properties you want in terms of strength.

If you make a model kit with biodegradable plastic but you have to buy 5 of them instead of 1 because they keep breaking, using ABS is more sustainable in my opinion.